As mentioned, according to the Ashkenazic minhag, and of some Sephardim, Nefillat Apayim is performed by lowering one’s head and leaning it on the arm. In the opinion of the Shulchan Aruch, one always falls on his left arm. According to the Rama, in the morning, when one’s tefillin is placed on his left arm, he falls on his right arm, and at Minchah, he falls on his left arm – this is the Ashkenazic custom (Shulchan Aruch and Rama 131:1; Mishnah Berurah 6).
When falling on the left arm, we tilt our faces slightly to the right, so as not to point them straight down to the floor. Similarly, when we fall on the right arm, we tilt our faces slightly to the left. We practice this just as it was practiced when people were accustomed to actually prostrating themselves on the ground, for in those times, they tilted their faces as a “fence” against the prohibition of prostrating oneself on a stone floor (Mishnah Berurah 131:40; Bei’ur Halachah 131:1).
It is customary to cover one’s face with clothing. It is not sufficient to conceal one’s head with his arm, since the arm and the face are one body, and the body cannot cover itself (Mishnah Berurah 131:3). The main purpose of this covering is for the sake of modesty, like that of a person who hides his face from Hashem out of trepidation and shame. One who is wearing short sleeves, and has a handkerchief, should place it on his arm and place his face on it. However, if he does not have a handkerchief, he may fall on his bare arm, but not on his palm, since it is impossible to hide one’s face with one’s palm. If there is a table there, he rests his arm and head on it, and the table is considered the main cover for his face.
It is customary to perform Nefillat Apayim only in places where a Torah scroll is present, or even other printed sifrei kodesh (sacred texts). In a place in which there are no sacred texts, the prayer is recited while sitting, without falling on one’s arm.
When Nefillat Apayim is performed in the rooms adjacent to the synagogue, which do not contain a Torah scroll or sacred books, if it is possible to see the aron kodesh (holy ark) from there, one falls on his face. However, when the aron kodesh cannot be seen, the prayer is recited while sitting.
In Jerusalem, it is customary to perform Nefillat Apayim even in a place without sacred texts, since the sanctity of the city serves as a substitute for the texts.
In a place where it is impossible for someone to recite the Nefillat Apayim prayer while sitting, such as in a place without a chair, or in a place in which another person is praying the Amidah directly behind him and he cannot go elsewhere, he may stand (Mishnah Berurah 131:10). It is best that one leans against a wall in such a way that without it he would fall, so that his prayer is considered to be recited partially sitting and partially in the position of Nefillat Apayim (Kaf HaChaim 38).
. The Bei’ur Halachah 131:1 clarifies that with the way we fall on our faces today, there is no concern of transgressing the prohibition of prostrating oneself on a kneeling stone, since our heads are very far away from the floor. Therefore, even if the floor is made of stone, it is not forbidden, since there are two differences here from the biblical prohibition: 1) this is not prostration, and 2) one’s face is not touching the ground. If so, the custom to lean on one’s side exists to remember the minhag when people were accustomed to bowing down completely on the ground, and if the floor was stone, they would have to bow with their head to the side. According to this, even the covering of one’s face does not serve as a separation between one’s face and the ground; rather the falling on one’s face is to hide his face in shame. Perhaps even the covering of oneself with clothing is to remember the fact that they used to bow down completely on the ground, for then if it was a stone floor, one could either turn his face or make a separation between himself and the floor. However, from the Magen Avraham 131, paragraphs 2 and 20, it can be inferred that even when one’s face is far from the ground, it is still considered bowing down, and therefore he is obligated to lean on his side or make a separation between his face and the ground. The Bei’ur Halachah questions this, and it is his view that nowadays, the custom to lean exists only to remember the custom of bowing down.
If so, when one does not have a sleeve or piece of fabric, it seems that he may cover his face with his bare arm, for not having fabric does not prevent one from fulfilling his obligation. If there is a table or lectern (shtender) there, it is best that one rest his head on his arm far into the depth of the table, so that the table can also be considered a cover for his face. Some lean on their watches and use them as a separation, though there is no reason for this, since a watch cannot cover one’s face. Even according to the Magen Avraham it is not considered to be a separation due to its small size. On the contrary, the kabbalists maintain that one must not lean on the palm of his hand at all, for on it are recorded all of his sins (Kaf HaChaim 131:47; Piskei Teshuvot, note 34), and one’s watch is close to his palm.
See Piskei Teshuvot 131, note 31, where he writes that according to the Magen Avraham, who maintains that the cover is intended to separate between one’s face and the floor, it is not enough to rest one’s forehead on his sleeve, for if he does, he will be facing the floor without a separation. Instead, one must cover his face with his arm. If there is a table there, he may lean his forehead on his arm and then the table is considered a separation between his face and the floor. Further, according to the Magen Avraham, it is permissible to hold a siddur between one’s face and the ground which would effectively divide between them. Still, we learned that the Mishnah Berurah rules that there is no need for a divider here; rather it is merely a continuation of a custom from the time that people actually bowed down on the floor. However, concerning a covering, it is, indeed, proper that the main part of one’s face be covered.
. The Roke’ach 324 writes that Nefillat Apayim is only performed in a place where there is a Torah scroll, but the Beit Yosef questions his words. The Chida concludes from this that it is the opinion of the Shulchan Aruch that one may fall on his face even in a place where there is no Torah scroll. (In practice, however, many Sephardim are accustomed to not falling on their faces at all.) The Rama 131:2 rules like the Roke’ach. In the rooms adjacent to the synagogue, if the aron kodesh can be seen, one falls on his face (Mishnah Berurah 13). Even sifrei kodesh can be considered like a Torah scroll according to the majority of Acharonim, as written in Siddur Olat Ra’ayah p. 302, paragraph 4.
.According to Rav Shlomo Zalman Auerbach, this only applies to the Old City. Still, Rav Tikochinsky writes this about the entire city of Jerusalem, as is written in Siddur Olat Ra’ayah p. 302. Since we learned that there are poskim who completely disagree with the Roke’ach, and according to them there is no need for a Torah scroll at all, in every case of uncertainty concerning this law, it is permissible to follow those who maintain that one may fall on his face, especially since that is the opinion of the majority of poskim.